The present invention relates to various methods and apparatuses for treating comminuted cellulosic fibrous material during the pulping process with a solution containing additives for improving the efficiency of the pulping process or for improving the quality of the pulp produced. Typical additives include, but are not limited to, polysulfide, sulfur and sulfur-containing compounds (e.g. hydrogen sulfide), surfactants, and anthraquinone and their equivalents and derivatives. In the following discussion it is to be understood that use of the term "anthraquinone" is meant to encompass all anthraquinone-based chemicals, their equivalents and derivatives.
Paper products today are manufactured from cellulose pulps produced by a variety of methods. For example, newsprint is made from a high-yield mechanical process in which the wood is ground to produce a pulp which retains 80% or more of the original constituents of the wood, including the undesirable, color-degrading and strength-diminishing constituents, for example, lignin. Fine papers of high brightness and cleanliness used for writing papers or food containers, for example, are typically made by chemical treatment in which the undesirable non-cellulose constituents of the wood, for example, lignin, are dissolved through chemical action typically under pressure and temperature, to produce a relatively pure form of cellulose fibers from which, for example, fine papers can be made. However, because the cellulose and non-cellulose constituents are not segregated in the wood and are typically intermingled with each other, it is difficult to dissolve the non-cellulose constituents without dissolving some of the cellulose. As a result, in the chemical treatment of wood, though the original wood may typically comprise or consist of 70 to 80% of the desirable cellulose and hemicellulose--(that is, the usable carbohydrates), typically only about 60 to 70% of the usable carbohydrates are retained in the final product. Some of the desirable carbohydrates are dissolved at the same time as the undesirable non-carbohydrate material. The percentage, by weight, of the amount of cellulose (and some non-cellulose) retained, excluding moisture, compared to the amount of wood introduced to the process is referred to as the "yield" of the process. Where mechanical pulping methods may have yields greater than 80%, chemical pulping processes typically have yields of about 50%. Of course, the paper manufacturer desires the highest yield possible.
In addition to yield, another important property of cellulosic pulps is the relative strength of the paper produced from the pulp. Typically, the strength of a paper is a function of two features of the cellulose fibers from which the paper is produced: the intrinsic strength of the fibers and the strength of the bonds between the fibers. The strength of individual fibers is typically characterized as the amount of load that the fiber can withstand while under axial tension and also the amount of load that the fiber can withstand when exposed to a transaxial force, that is, shear. The strength of individual fibers is typically associated with what is termed the "tear strength" of a sample of paper produced from the fiber. The strength of the bonds between fibers is a function of the relative surface area and the flexibility of the fiber, among other things. The strength of these bonds is typically indicated by what is called the "tensile strength" of a sample of paper produced from the fibers. The tear and tensile strength of a paper sample are typically inversely proportional: as the tear strength increases, the tensile strength decreases, and vice versa.
The kraft chemical pulping process (also known as the sulfate process) is typical of a chemical pulp process that produces pulps of high strength and yields of around 50%. In the kraft process the wood is chemically treated under temperature and pressure with an aqueous solution of sodium hydroxide [NaOH] and sodium sulfide [Na.sub.2 S]. However, it is sometimes possible to incrementally increase the yield of the kraft process by introducing additives or chemical treatments to the process, typically before treatment with the sulfide and hydroxide. Note that a 1% increase in yield for a typical 1000 ton-per-day pulp mill, which sells pulp at approximately $500.00 per ton, can mean over 3 million dollars in added revenue per year, with no increase in wood usage. Thus, single-digit increases in yield can have significant impact upon the profitability of a pulp mill. If a pulp mill is capacity limited due to limitations in increasing the capacity of its recovery boiler, an increase in the yield of a pulping process can increase the capacity of the mill while avoiding the limitations of the recovery system.
As described in Pulping Processes (1965) by Rydholm [pp. 1003-1004] and elsewhere, it is generally understood that cellulose degradation under alkaline conditions is governed by what are referred to as "peeling" reactions and "stopping" reactions. Peeling reactions are the reactions that occur at the ends of cellulose molecules in which individual carbohydrate units, or monomers, are detached or "peeled" from the end of the carbohydrate chain. In this reaction, the aldehydic end groups of the cellulose chains are cleaved from the chain exposing a new aldehydic end group. This newly-exposed end groups can continue to be cleaved until a carboxyl end group is formed and the peeling reaction is terminated. This formation of a carboxyl end group is referred to as the "stopping" reaction. This stopping reaction stabilizes the carbohydrate chain against further degradation by "peeling". As described by Rydholm, typically 50 or more monomers are "peeled" from a newly-exposed end of a carbohydrate chain during alkaline chemical treatment. This degradation of the cellulose molecular chains can be manifest as a reduction in yield (that is, "peeling" causes the dissolution and loss of cellulose).
Conventional mechanisms for increasing the yield of chemical pulping process are directed toward limiting the amount of cellulose lost through alkaline peeling by promoting the stabilization of the end groups against this peeling reaction, that is, they promote the formation of a carboxylic end group.
As explained, for example, in Pulp and Paper Manufacture, Volume 5: Alkaline Pulping", edited by Grace, et al. [pp. 114-122], several recognized additives can be used to stabilize the alkaline peeling reaction and incrementally increase the yield of chemical pulp mills. These include sodium borohydride [NaBH.sub.4 ], sodium polysulfide [Na.sub.2 S.sub.n ] (known simply as "polysulfide"), and anthraquinone (AQ). Smook (1989) in his Handbook of Pulp and Paper Technologists also mentions that hydrogen sulfide [H.sub.2 S] gas pretreatment of chips can be used to increase yield.
U.S. Pat. No. 4,012,280 discloses that improved yield of an alkaline chemical pulping process can be obtained by adding cyclic keto compounds, including anthraquinone, to the cooking liquor and treating cellulose material with the cooking liquor-AQ solution at pulping temperatures. However, in such a process the AQ additive is not recovered and is simply lost to the pulping process, even though it is known that AQ is a catalyst. U.S. Pat. No. 4,127,439 improved on the earlier AQ treatment process by limiting the exposure of cellulose material to AQ only in a pretreatment stage prior to digestion. In this process, the pretreatment liquor is separated from the cellulose material prior to digestion and the separated pretreatment liquor containing residual AQ is re-used for pretreatment. U.S. Pat. No. 4,127,439 includes the option of pretreating cellulose in a continuous process in which the treatment liquid counter-currently displaces the pretreatment liquor in a single treatment zone. However, the removal and recovery of the pretreatment liquor is limited due to the treatment in one treatment zone.
U.S. Pat. No. 4,310,383 discloses an alternative to the above pretreatment with anthraquinone in which the variation in the solubility of the anthraquinone in an alkaline liquor is used to produce an internal circulation of anthraquinone in a treatment zone. This internal circulation results from the variation in the solubility of anthraquinone which occurs in a counter-current treatment of cellulose. The AQ-containing solution is introduced at one end of a counter-current treatment zone at higher alkalinity where the AQ is more soluble. This high alkalinity is effected by also introducing highly-alkaline kraft white liquor while introducing the AQ to the cellulose. The alkalinity of the counter-current flowing liquid decreases as the alkali is consumed by the cellulose material such that the alkalinity of the AQ solution is reduced to a point where the AQ becomes insoluble and precipitates onto the cellulose. The down-flowing cellulose then carries the precipitated AQ back into the other end of the treatment zone where the alkalinity is higher such that the AQ again dissolves. The dissolved AQ then passes back counter-currently to the flow of cellulose and the cycle repeats itself. Though this process provides for the recovery and re-use of anthraquinone it is not applicable to treatments with other additives, such as polysulfide or sulfur, which are not characterized by such variation in solubility due to alkalinity.
The present invention comprises or consists of a process of producing cellulose pulp from cellulose material with the aid of a strength or yield-enhancing additive in a manner such that the additive is more effectively used and the loss of the additive is minimized. Contrary to the process described in U.S. Pat. No. 4,310,383, the present invention is not dependent upon alkalinity and its effect upon the solubility and precipitation of the additive. The present invention is based upon the natural mass transfer of chemical additives from solution to the carbohydrates, the effect of liquid flows on this mass transfer, and the efficient recovery and reuse of the additives. This process is particularly amenable for use with the process and equipment described in the following U.S. Pat. Nos. 5,489,363; 5,536,366; 5,547,012; 5,575,890; 5,620,562; 5,662,775 and others, and sold by Ahlstrom Machinery, Glens Falls, N.Y., under the trademark LO-SOLIDS.RTM.. That is, the present invention is most amenable to conditions under which the concentration of dissolved organic material in the treatment liquor is minimized, as is characteristic of the Lo-Solids.RTM. processes available from Ahlstrom Machinery Inc. of Glens Falls, N.Y.
The process and equipment of the present invention, marketed by Ahlstrom Machinery under the trademark LO-SOLIDS.RTM.-M2.TM., utilizes the flexibility of a LO-SOLIDS.RTM. configuration in order to enhance the effectiveness of additives such as anthraquinone, polysulfide, sulfur and sulfur-containing compounds, surfactants, or any combination thereof. It is designed to maximize additive concentrations and retention times. It is also designed to optimize the additive concentration profile with respect to the alkali concentration, dissolved organic material concentration, and temperature profiles of the cook.
First, consider factors which influence the effectiveness of chemical additives in the pulping process, for example, anthraquinone. During kraft pulping, anthraquinone will oxidize the reducing-end groups of polysaccharides into alkali stable carboxylic acids. This stabilization arrests alkali-peeling reactions and thus results in increased polysaccharide yield. The reduced form of anthraquinone then reacts with lignin. Reactions with lignin render the lignin more prone to degradation and dissolution, and also serve to re-generate the oxidized form of the anthraquinone. Thus, anthraquinone is a catalyst which performs two useful functions in kraft cooking: (i) it stabilizes polysaccharides thus enhancing yield, and (ii) it accelerates delignification.
A true catalyst is not consumed during a reaction and so its effectiveness will depend primarily on "activity" (or concentration) within the reaction mixture. The concentration of anthraquinone will depend on: (i) the amount of anthraquinone added to the system, and (ii) the hydraulic liquid-to-wood (L:W) ratio in the system. For a continuous digester, the L:W ratio varies from zone to zone. For a LO-SOLIDS.RTM. operation, the L:W varies more than for a conventional system and it can be independently controlled from zone to zone.
Unfortunately, all of the prior art literature on AQ is reported on a %-applied-on-wood basis and does not, therefore, take into account hydraulic and concentration effects. The prior art entirely comprises results for conditions which are typical of a batch lab or a batch full scale process: specifically, for conditions where the L:W ratio is greater than 3.5:1 (typically 4:1 or more). For a conventional continuous digester, however, the L:W ratio in the impregnation zone will be somewhere between 2.5 and 3.5 to 1. Thus, the concentration and effectiveness of anthraquinone or other additives will be as much as 35% greater in a conventional continuous process than the literature would suggest.
For modified cooking processes such as LO-SOLIDS.RTM. pulping, a large portion of the white liquor is shifted away from the feed and introduced, instead, straight into the digester. This means the initial L:W ratio in the impregnation zone will be less than the initial L:W ratio of conventional, non-modified systems. As a result, the concentration of additives, such as anthraquinone, will be greater in a modified system, if the % applied to the feed remains constant.
One hindrance to the understanding of the effect of chemical additive concentration on the effectiveness of the treatment is the conventional nomenclature used to describe the amount of liquid present in a cooking process. As discussed above, the expression "liquid-to-wood ratio" or "liquor-to-wood ratio" is commonly used in the art to indicate how much liquid is present relative to the amount of wood or cellulose. In batch processes, in which wood and liquids are introduced in discreet amounts and are retained in an enclosed vessel, these ratios provide somewhat useful information. However, in continuous processes, especially in modified continuous processes in which liquids may flow independently of the wood material, the amount of liquid and wood present in a region of the digester is not as well defined. For example, a slurry of chips and liquid flowing through a continuous treatment vessel contains some liquid that is trapped within the pores of the chips, that is, the so-called "bound" liquid, and some liquid which is "free" to flow about the chip. Though the amount of "bound" liquid may remain relatively constant, the volume of "free" liquid may vary depending on the flow direction and flow rate of the liquid in the digester. Furthermore, the amount of "wood" present during different stages of a continuous cooking process varies as the pulping process progresses. More wood is present earlier in the process than in the later stages of the process. Thus, defining a quantity "per wood" is also somewhat ambiguous.
Thus, unlike the batch process, a "liquor-to-wood" ratio for a continuous pulping process may be misleading, or at least not completely representative of the conditions that are present in a continuous digester, especially a digester in which the concentration of chemical additives in the liquid is under consideration.
In order to better define the conditions that exist within a continuous digester and to better understand the significance of the present invention, the following terms have been coined, and are defined as follows: the Net Liquid Flow Rate (NLFR) and the Net Additive Concentration (NAC). The NLFR is the vector sum of the volumetric flow rates of the bound liquor, F.sub.B, plus the volumetric flow rate of the free liquor, F.sub.F, using the convention that the direction of the bound liquid flow is positive. That is, a treatment region having a co-current flow of treatment liquid will have an NLFR given by: EQU NLFR=F.sub.B +F.sub.F (1)
while a region having a counter-current flow of treatment liquid will have an NLFR given by: EQU NLFR=F.sub.B -F.sub.F (2)
An NLFR may be expressed in any preferred volumetric flow dimensions, for example, gallons per minute (gpm) or liters per minute (lpm), but NLFR is preferably expressed in units of "tons of liquid per ton of wood fed to the system", or T/T. As indicated by equation (2), an NLFR may be positive or negative. In the present invention, the NLFR may range from -2 to 6 TIT, but is preferably between -1 and 3 T/T, and may vary between different treatment zones. The NLFR provides a more useful parameter for characterizing the liquid flow rates through a treatment zone of a continuous digester than the more conventional liquor-to-wood ratio.
The Net Additive Concentration (NAC) of a chemical additive is simply the specific concentration of the additive chemical present in the liquor flowing through a treatment zone, that is, the additive concentration present in the NLFR. This concentration is determined by dividing the mass flow rate of the additive introduced into the treatment zone by the NLFR present in the treatment zone, that is, EQU NAC=[Grams/minute of additive]/NLFR (3)
Thus, NAC can typically be expressed as pounds per gallon or grams per liter of additive present in the treatment zone. In a preferred method, the additive flow is expressed in "tons of additive per ton of wood fed to the system", such that a equation (3) yields the dimension "tons of additive present in a treatment zone per ton of liquid present". Note that if NLFR is negative, that is, the treatment zone is a counter-current treatment zone, the absolute value of NLFR can be used.
As an example, the typical conventional amount of AQ charged to a system is about 0.1% maximum, or 0.001 tons of additive per ton of wood fed to the system (T/T). Due to deactivation, consumption, and other factors, in prior art systems, this charge typically produces AQ concentrations in the treatment liquor of approximately 0.00075 T/T, typically less than 0.0010 T/T. However, the NAC present in the treatment zone of the present invention can exceed 0.0015 T/T and even exceed 0.0020 T/T, while not increasing the 0.1% charge of AQ. The value of NAC will vary for other additives. For example, since the maximum charge of polysulfide is about 1% on wood, or 0.01 T/T, the NAC for polysulfide for the present invention is expected to be about 10 times that of AQ.
The NAC calculated by equation (3) is the average concentration of the additive in the treatment zone. The actual local concentrations will vary due to the variation in the flow through the zone and additive concentration gradients, due to decomposition and deactivation, within the zone. The present invention maximizes the NAC in a treatment zone of a continuous digester by minimizing the NLFR in the treatment zone.
It is known that the presence of dissolved organic material (for example, dissolved lignin, dissolved cellulose, and dissolved hemicellulose, among other dissolved wood materials) interferes with the effectiveness of additives. For example, dissolved lignin deactivates anthraquinone such that it is less effective in preserving yield during a pulping process. Another feature of the present invention is that the concentration of additive present during treatment is increased while concentration of dissolved organic material, which can interfere with the beneficial effects of the additive, is minimized during treatment with an additive such that the effectiveness of the treatment is optimized.
One method of expressing this optimized condition is by use of the ratio of the concentration of additive, [A], to the concentration of dissolved organic material, [DOM]. This ratio, referred to as the "M2 Ratio", is given by:
M2 Ratio=[A]/[DOM] (4)
where the concentration of the additive, A, is expressed in milligrams per liter (mg/l) and the concentration of DOM is given as grams per liter (g/l). For example, at a point in the treatment where the average anthraquinone concentration is 200 mg/l and the average DOM concentration is 100 g/l the M2 ratio is 2.0 mg/g. Specifically, this is referred to as the "M2-AQ ratio", since the additive is anthraquinone. In the prior art, using a maximum AQ charge of 0.1%, the concentration of AQ in the treatment liquid, due to deactivation and consumption, is typically less than 300 mg/l and the concentration of dissolved organic material in the same treatment liquor is typically 100 g/l or more. Thus, in prior art treatments with AQ the ratio of the concentration of the AQ to the concentration of the DOM is typically less than 3.0 mg/g. Typical values for the M2-AQ Ratio according to the present invention, for an AQ charge of 0.1%, are at least 4.0 mg/g, preferably, at least about 5.0 mg/g, most preferably, at least about 6.0 mg/g, and sometimes over 8.0 mg/g.
Other ratios are defined for other additives, such as the "M2-PS ratio" for use when polysulfide is the additive, or the "M2-Surf Ratio" when surfactants are used. For example, since the typical charge of polysulfide is about 10 times that of anthraquinone, the value of the M2-PS Ratio is expected to be about 10 times that of M2-AQ, or at least about 40.0 mg/g, preferably, at least about 50.0 mg/g, most preferably, at least about 60.0 mg/g. [Thus an M2-AQ Ratio of 5.0 mg/g is equivalent to an M2-PS Ratio of about 50.0 mg/g.]
According to the present invention, it is desirable to have the highest practical additive concentration while having the smallest DOM concentration. Thus, according to the present invention, the highest M2 ratio possible is preferred.
Typically, additives such as polysulfide, anthraquinone, and the like, are removed from the cooking vessels with the liquors through one or more conventional annular screen assemblies. This liquid containing valuable additives is typically either recirculated back to the cooking vessel via a circulation or forwarded to the chemical and heat recovery system of the pulp mill. In either case, the valuable additive may be lost from the process and therefore must be replenished with a fresh supply of additive if treatment is to continue. The present invention also includes the method of recovering at least some of the additive in the liquor removed from the digester by passing the additive-bearing liquid through one or more filtration devices, preferably an ultra-filtration device. This may require that the liquid stream be cooled prior to introducing it to the filtration device, for example, by conventional evaporation or flash evaporation or passing the liquor through a heat exchanger. The additive separated from the liquor can be reintroduced in the process as needed, for example, as a supplement to the fresh additive that is introduced. Anthraquinone is one additive that can be recovered and re-used in this manner.
In its simplest form, the process of the present invention comprises or consists of the following: (a) treating (e.g. pretreating) the cellulose material with a solution containing a yield or strength-enhancing additive; (b) displacing the majority, preferably the vast majority (typically over about 90%), of any of the additive from the cellulose material prior to bulk delignification in a counter-current treatment zone so that the content of the additive in the material slurry is minimized; and (c) treating the material with an alkaline cooking liquor to produce a cellulose pulp. Preferably (a) is performed in a counter-current fashion. The additive used in (a) is preferably anthraquinone or its equivalents or derivatives (collectively "AQ"), but other additives such as polysulfide, hydrogen sulfide, a surfactant (for example, a surfactant can be used with anthraquinone to enhance the solubility of the anthraquinone), sulfur or sulfur-containing compounds, or others, or combinations thereof, or combinations thereof in the presence of a cooking liquor, such as kraft white, green or black liquor, may be utilized.
Preferably (a) is performed at a temperature and alkalinity at which little or no additive is consumed and is thus available for recovery at (b) and can be re-used. The temperature of treatment (a) is preferably below cooking temperature, typically below 140.degree. C., for example, between about 120 and 140.degree. C., preferably between about 125.degree. and 140.degree. C. The temperature during (b) is typically between about 130-150.degree. C., preferably, between 130 and 145.degree. C. The NLFR during (a) is typically between -2.0 and 2.0 T/T, preferably, between about -1.0 and 1.0 T/T, most preferably, between about -0.5 to 0.5 T/T, or as close to 0 as practical. The NLFR during (b) is typically between -3.0 to 1.0 T/T, preferably, between about -3.0 to 0 T/T, most preferably between about -2.0 and -1.0 T/T.
Since the principal treatment with additive occurs during (a) it is desirable to establish the highest possible Net Additive Concentration (NAC) during (a). According to the present invention the NAC during (a) is at least 0.0010 T/T, preferably at least about 0.0015 T/T, most preferably at least about 0.0020 T/T. Since during (b) the additive is being displaced it is preferable to have the least additive possible present during (b). Also, the M2 Ratio during (a) is also preferably as high as possible. For example, the M2-AQ Ratio during (a) is typically at least 4.0 mg/g, preferably, at least about 5.0 mg/g, most preferably, at least about 6.0 mg/g. The M2-PS Ratio during (a) is typically at least about 40.0 mg/g, preferably, at least about 50.0 mg/g, most preferably, at least about 60.0 mg/g. Again, the M2 Ratios during (b) are preferably as small as possible since the additive is being displaced.
The alkali concentration, or effective alkali, in (a) typically ranges from 3 to 14 g/l expressed as NaOH, for example, the alkali concentration at the beginning of (a) may be about 3 to 6 g/l as NaOH and the alkali concentration at the end of (a) may be about 10 to 14 g/l as NaOH. The alkali concentration in (b) typically ranges from about 6 to 18 g/l as NaOH, for example, the alkali concentration at the beginning of (b) may be about 6 to 8 g/l as NaOH and the alkali concentration at the end of (b) may be about 14 to 18 g/l as NaOH.
Preferably (c) comprises or consists of a co-current or counter-current cooking process, for example, the LO-SOLIDS.RTM. cooking process described in the above-referenced U.S. patents. The alkaline cooking liquor of (c) is typically kraft white liquor, green liquor, or black liquor, or soda cooking liquor, or a polysulfide containing liquor, or some combination thereof. Preferably (c) is performed at a temperature of at least 140.degree. C., typically, between about 140 and 160.degree. C. and at an effective alkali concentration of greater than 15 g/l, expressed as NaOH, typically, between about 17 to 23 g/l expressed as NaOH; and (a)-(c) are preferably practiced so as to provide a yield of at least 3% (e.g. at least 4 or 5%) higher than the yield produced by methods not employing (a) and (b).
In a preferred embodiment of the present invention, (a) is preceded by (d) pretreating the cellulose material with an alkaline liquid, with or without the presence of an additive. Preferably (d) is a co-current treatment, though it may also be counter-current treatment, and is performed at a temperature less than 130.degree. C., preferably less than 120.degree. C., for example, between about 100 and 110.degree. C., for example, (d) may be an impregnation or a cool impregnation. Furthermore, (d) is preferably followed by (e) removing at least some of the free liquor from the slurry prior to (a). Preferably (e) is a post-impregnation extraction which removes the dissolved organic material produced during (d) such that the concentration of dissolved organic material is minimized prior to (a). Some of the liquid removed during (e) may contain useful additive; this liquid may be re-introduced to the cellulose material prior to or during (d).
In another embodiment, a further step (f), prior to (c), is performed in which at least some of the liquid in the material slurry is removed from the slurry after (a). This liquid, which typically contains at least some additive, may be re-introduced to the slurry prior to or during (d). Also, (f) may be performed after (b) and the liquor removed re-introduced prior to or during (d).
According to another aspect of the invention a method of continuously producing chemical cellulose pulp from comminuted cellulosic fibrous material slurry, with a yield or strength increase, is provided comprising: (a) Treating (e.g. pretreating) the comminuted cellulosic fibrous material slurry with a solution containing yield or strength-enhancing additive. (b) Displacing liquor containing at least some of the additive from (a) in a continuous counter-current treatment zone. (c) Recirculating liquor containing displaced additive from (b) to the slurry in (a). And, (d) treating the material with an alkaline cooking liquor, at cooking temperature, to produce a cellulose pulp with higher yield or strength than if (a) and (b) were not practiced.
In the method (a) is practiced using AQ, PS, NaBH.sub.4, sulfur or sulfur-containing compounds, a surfactant, combinations thereof, or combinations thereof with other chemicals. Also preferably in the method (a) is practiced using AQ, or a combination of AQ and other chemicals; and (a) is practiced at a temperature below about 140.degree. C., for example, between about 120 and 140.degree. C., preferably between about 125.degree. and 140.degree. C., and (b) is practiced at a temperature of between about 130-150.degree. C., preferably, between about 130 and 145.degree. C. The NLFR during (a) is typically between -2.0 and 2.0 T/T, preferably, between about -1.0 and 1.0 T/T, most preferably, between about -0.5 to 0.5 T/T, or as close to 0 as possible. The NLFR during (b) is typically between -3.0 to 1.0 T/T, preferably, between about -3.0 to 0 T/T, most preferably, about -2.0 and -1.0 T/T. The NAC and M2 ratios during (a) and (b) are typically as discussed previously.
The alkali concentration, or effective alkali, in (a) typically ranges from about 3 to 14 g/l expressed as NaOH, for example, the alkali concentration at the beginning of (a) may be about 3 to 6 g/l as NaOH and the alkali concentration at the end of (a) may be about 10 to 14 g/l as NaOH. The alkali concentration in (b) typically ranges from about 6 to 18 g/l as NaOH, for example, the alkali concentration at the beginning of (b) may be about 6 to 8 g/l as NaOH and the alkali concentration at the end of (b) may be about 14 to 18 g/l as NaOH.
According to another aspect of the invention a continuous digester system is provided comprising: A substantially vertical digester vessel having a top and a bottom. An inlet for comminuted cellulosic material liquid slurry, adjacent the vessel top. An inlet for yield or strength-enhancing additive in the upper half of the vessel. An outlet for chemical pulp adjacent the vessel bottom. A liquor/material separator adjacent the inlet for separating some liquid from the slurry introduced through the inlet. A first set of screens at a first vertical level in the digester, below the separator. A second set of screens at a second vertical level in the digester below the first set. A third set of screens at a third vertical level in the digester, below the second set. Means for recirculating liquor containing displaced yield or strength containing additives from the first set of screens to the slurry above the first set of screens. Means, including the first set of screens, for establishing a counter-current, upward, flow of liquid substantially between the first and second set of screens in a first zone. And, means for introducing yield or strength-enhancing additive into the vessel adjacent the second set of screens to flow upwardly with liquid in the first zone.
The digester system preferably also is constructed so that the first set of screens comprises a top screen and a bottom screen; and wherein the reintroducing means comprises the bottom screen and a conduit leading from the bottom screen to the slurry before or after the inlet; and wherein a conduit from the top screen is connected to a flash tank. Also the system preferably further comprises means, including the second set of screens, for providing a counter-current flow of liquor to slurry in a second zone, between the second and third set of screens. Also, preferably the third set of screens comprises a top third screen and a bottom third screen; and the system further comprises a conduit from the bottom third screen connected to a flash tank, and a conduit from the top third screen returning liquor to the interior of the vessel adjacent the third set of screens.
The relationship of the recirculation conduit outlets to the screen assemblies in the present invention are also preferably as disclosed in U.S. Pat. No. 5,849,151, the disclosure of which is incorporated in its entirety in this specification.
It is the primary object of the present invention to produce chemical cellulose pulp (e.g. kraft pulp) with enhanced yield and/or strength in a relatively cost effective manner since yield and/or strength enhancing additives are almost completely effectively use, rather than being destroyed as in the prior art. This and other objects of the invention will become clear from the detailed description of the invention and the appended claims.